Group 7 (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

The halogens

What are the halogens?

• The elements in Group 7 are known as the halogens:

  • Fluorine

  • Chlorine

  • Bromine

  • Iodine

  • Astatine

• They are non-metals and are all poisonous

• All halogens have 7 electrons in their outer shell, which gives them similar chemical properties

• Halogens are diatomic

  • They exist as molecules made of pairs of atoms joined by a single covalent bond (F₂, Cl₂, Br₂, I₂)

• When halogen atoms gain an electron during reactions, they form 1− ions called halide ions

Electronic structures of the Group 7 elements

States and appearance at room temperature

• At room temperature (20 °C), the physical state of the halogens changes as you go down the group

  • Fluorine and chlorine are gases

  • Bromine is a liquid

  • Iodine is a crumbly solid

• The colours of the halogens also change as you descend the group

  • They become darker

The appearance and state of the Group 7 elements

The physical states and colours of chlorine, bromine and iodine at room temperature 

• Chlorine, bromine and iodine react with metals and non-metals to form compounds

Reactions with metals

• Halogens react with metals to form ionic compounds called metal halide salts

• The halide ion carries a 1− charge

  • So, the number of halogen atoms in the compound depends on the charge of the metal ion

• Group 1 metal example:

2Na + Cl₂ → 2NaCl

• Group 2 metal example:

Ca + Br₂ → CaBr₂

• The halogens decrease in reactivity moving down the group, but they still form halide salts with some metals including iron

• The rate of reaction is slower for halogens which are further down the group such as bromine and iodine

Reactions with non-metals

• The halogens react with non-metals to form simple molecular covalent structures

• For example, the halogens react with hydrogen to form hydrogen halides:

H₂ + Cl₂ → 2HCl

H₂ + Br₂ → 2HBr

• Reactivity decreases down the group

  • Fluorine is the most reactive, reacting with hydrogen at low temperatures in the absence of light

  • Chlorine reacts with hydrogen but requires light or a high temperature

  • Iodine reacts less vigorously with hydrogen

Group 7 Trends

Melting and boiling points

• The melting and boiling points of the halogens increase going down the group

• The increase in melting and boiling point is because:

  • The atoms become larger going down the group

  • So, the intermolecular forces increase

  • These increased forces require more energy to overcome

  • This results in higher melting / boiling points

Reactivity trend

• Reactivity decreases going down Group 7

• Halogens react by gaining an electron to achieve a full outer shell

• Going down the group:

  • Atoms have more electron shells

  • So, the outer shell is further from the nucleus

  • The attraction between the nucleus and an incoming electron is weaker

  • This makes it harder to gain an electron, so reactivity decreases

• Fluorine is the most reactive halogen; iodine is the least reactive of the three commonly tested

Halogen displacement reactions

• A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide

• You only need to learn the displacement reactions with chlorine, bromine and iodine

• Reactivity order:

(most reactive) chlorine > bromine > iodine (least reactive)

Chlorine with bromide & iodide solutions

• If you add chlorine solution to colourless potassium bromide solution:

  • A displacement reaction occurs:

  • The solution becomes orange as bromine is formed

• If you add chlorine solution to colourless potassium iodide solution:

  • A displacement reaction occurs:

  • The solution becomes brown as iodine is formed

• Chlorine is above bromine and iodine in Group 7 so it is more reactive

• Chlorine will displace bromine or iodine from an aqueous solution of the metal halide

chlorine + potassium bromide →  potassium chloride + bromine

Cl₂ + 2KBr → 2KCl + Br₂

chlorine + potassium iodide →  potassium chloride + iodine

Cl₂ + 2KI → 2KCl + I₂

Bromine with iodide solution

• Bromine is above iodine in Group 7 so it is more reactive

• Bromine will displace iodine from an aqueous solution of the metal iodide

bromine + potassium iodide →  potassium bromide + iodine

Br₂ + 2KI → 2KBr + I₂

Summary table

Ionic equations

Higher tier only

• For Higher Tier, you can be asked to provide ionic equations for the halogen displacement reactions:

Cl₂ + 2Br^- → 2Cl^- + Br₂

Cl₂ + 2I^- → 2Cl^- + I₂

Br₂ + 2I^- → 2Br^- + I₂